In accordance with a typical prior art stretching process, such as, for example, the SIGNODE process, a cast sheet of thermoplastic material, such as, for example, polypropylene, is first reduced in size, that is, the thickness dimension thereof, by rolling the sheet through a pair of closely spaced milling rollers or cylinders which rotate in opposite directions. After the thickness of the sheet has been reduced, the sheet is then drawn and stretched out from the milling rollers by means of a series of orienting rollers or a bridle assembly so as to achieve its finalized desired size or thickness.
Another prior art process or method that is commonly used in connection with the fabrication of stretched sheet materials is a process known as or called the short-gap method and is generally comprised of an entry bridle, a stretching assembly, and an exit bridle In accordance with this process, a slow speed, heated entry bridle assembly advances a cast sheet of material, usually film, to a stretching assembly which comprises a pair of rollers or cylinders which are set a predetermined distance apart. The first roller rotates at the same speed as the entry bridle, whereas the second roller rotates at a speed which is greater than that of the first roller and which is equal to the rotary speed of the exit bridle. Thus, as the film passes through the entire assembly, it is stretched to its finalized desired size or thickness dimension.
These prior art methods or processes present several disadvantages. For example, the properties of the straps produced by these methods or processes provide or exhibit limited increases in strength without significant increases in other desired properties. In addition, substantial necking of the sheets occur as the sheets are stretched over the distance or space defined between the rollers.
The aforenoted parent patent application presented, disclosed, and embodied a novel process and apparatus for achieving the simultaneous milling and stretching of such sheets whereby the aforenoted problems were minimized, however, other operational problems with such process and apparatus have now been discovered which require rectification in order to achieve still better properties characteristic of such simultaneously milled and stretched sheets of thermoplastic material. For example, one operational problem or deficiency which has been discovered in connection with the fabrication or manufacture of the simultaneously milled and stretched sheets of thermoplastic material in accordance with the zero-gap process and apparatus set forth and described within the aforenoted parent patent application is that when the thermoplastic sheet is ready to be worked, that is, simultaneously milled and stretched, by being conducted through the nip defined between the oppositely rotating rollers of the zero-gap assembly, it has been found that the opposite surfaces of the thermoplastic sheet exhibit different surface temperature values. This is because as the thermoplastic sheet is routed about the first or upstream roller of the zero-gap roller assembly, the inside surface of the thermoplastic sheet, that is, the surface of the sheet which is in direct contact with the first or upstream roller of the roller assembly, will be heated by the first or upstream roller, or at least have its surface effectively prevented from being cooled by the ambient air because such surface is disposed in direct contact with the first or upstream roller and not exposed directly to the ambient air. On the contrary, the outer or exterior surface of the thermoplastic sheet, that is, the surface of the sheet which is not disposed in direct contact with the first or upstream roller, is effectively cooled as a result of being exposed to the ambient air. Such temperature differential between the aforenoted surfaces of the thermoplastic sheet leads to density differentials throughout the thermoplastic sheet which adversely affects the various properties of the processed sheets such as for example, the tensile strength, weldability and associated properties, and split resistance.
Another problem or deficiency which has been discovered in connection with the aforenoted zero-gap simultaneous milling and stretching apparatus and method is that as a result of the processing of the thermoplastic sheet, the opposite edge portions of the sheet become thickened, or in other words, the sheet does not exhibit uniform thickness or flatness across the width thereof. Consequently, when the processed sheet is subsequently desired to be processed further into thermoplastic strapping, the thickened edge portions cannot be used to fabricate such straps, at least without further processing in order to effectively reduce the thickness dimension to the desired thickness dimension characteristic of the useable strapping. Consequently, it is desired to laterally extend or increase the width of the zone or region of the processed sheet which exhibits the uniform thickness dimension or flatness of the sheet so as to effectively increase the amount of processed sheet which can then be readily fabricated into the desired strapping.